The monopile for offshore wind turbine sustains long-term cyclic load due to environment and scouring in the seabed. The long-term cyclic loading behavior of the pile foundation under scouring is an important problem in the design of monopile for an offshore wind turbine. Laboratory model tests were conducted to investigate the performance of a large diameter pile and a small diameter pile in sand subjected to scouring and long term lateral cyclic loadings, in which the influence of scour depth and cyclic loading amplitude on the long-term behavior of the two foundations was discussed. The test results show that the applied cyclic loading improves the post-cyclic capacity of single pile, especially for the small diameter pile. Scour leads to the reduction of post-cyclic capacity of single pile, the effect of which increases with the scour depth. Comparing with the large diameter pile, the increment of residual cumulative deformation of the small diameter pile is smaller in early cyclic loading stage. When the cycle number is larger than 1000 and the scouring depth is twice of the pile diameter, the development of the cumulative deformation of the small diameter pile approaches to be stable, while a further development of deformation appears in the large diameter pile. By bringing the scour affected capacity into the dimensionless cyclic loading amplitude parameters, the lateral cyclic cumulative displacement calculation model of the two kinds of piles under scouring is established, which is applicable to the prediction of long-term lateral cyclic cumulative deformation of single pile considering scouring.